Daten no Inugami: SLASHDØG - High School DxD Universe

Tobio Ikuse, a second-year high school student, missed a school trip on a luxury liner due to a severe fever. When he later learns that the teachers and second-year students from the trip have vanished, he is deeply affected. Months later, after gradually recovering, Tobio unexpectedly runs into one of his missing classmates, Sasaki. Pursuing him, Tobio is surprised to find Sasaki with a Lizard Monster that attacks him. He is saved by Natsume Minagawa, another survivor from the liner, who gives him an egg before leaving. Later, Tobio is attacked again, this time by two individuals with monsters. A puppy emerges from his shadow and helps fend them off. Natsume returns and decides to take Tobio to their hideout.